welding lecture by Jinu Paul from IITKharagpur

1. 9/20/2012
1
Casting, Forming & Welding
(ME31007)
Jinu Paul
11
J u au
Dept. of Mechanical Engineering
1
Welding Lecture ‐ 7
13 Sept 2012, Thursday 8.30 ‐9.30 am
Arc Welding-
Electrical features
22
2

2. 9/20/2012
2
Arc Electrical Features-impedance
• An electric welding arc is an impedance (related
to the resistance of a circuit, but including
contributions from capacitance and inductance
as well) to the flow of electric current
• Specific impedance at any point in an arc is
inversely proportional to the density of the
charge carriers and their inherent mobility.
• The total impedance depends on the radial and
33
p p
axial distribution of charge carrier density
• The impedance of the plasma column is a
function of temperature (except regions near the
arc terminals)
3
Arc-Electrical features Electrode
Plasma
Work
44
• All electric arcs consist of three regions
– the cathode fall space (or drop zone);
– the plasma column fall space (or drop zone)
– the anode fall space (or drop zone)
4

3. 9/20/2012
3
Arc-Electrical features
• Electrical power dissipated in each regions of
the arc given by P = I(Ea + Ec + Ep) where Ea is
anode voltage, Ec is cathode voltage, and Ep is
plasma column voltage
• Intermediate regions  Involved in expanding or
contracting the cross section of the gaseous
conductor to accommodate each of these main
i
55
regions.
• As a consequence, welding arcs assume bell or
cone shapes and elliptical or some other
noncylindrical contour.
5
Arc Shape
Electrode
Arc shape = Interaction of
(plasma + Arc + Ambience)
• Bell Shape
Plasma
66
Work
• Cone
• Elliptical
• Cylindrical

4. 9/20/2012
4
Arc Shape- Influencing factors
1. Shape of the arc terminals (i.e.,
pointed welding electrode
producing a narrow arc focused at
the electrode tip and flat work
piece electrode, which causes the
arc to spread)
2. Gravitational forces
3. Magnetic forces (
from both
77
g internally generated and externally
induced or applied sources)
4. Interactions between the plasma
and ambience (shielding gas)
7
Arc Shape- Influencing factors
• Nature of electrode- Consumable/non
consumable
• Electrode coating/gas generation
• Shielding gas
M itd & l it f t
8
• Magnitude polarity of current source

5. 9/20/2012
5
Arc Shape
(a) GTA (b) SMA
(c) CO2 shielded
GMA welding
Non
consumable
Consumable
l t d +
Consumable
99
9
electrode +
Inert gas
electrode Gas
generation
electrode +
CO2 gas
Arc radiation
• Arc radiation  amount and character
depends on
– the atomic mass and chemical composition of the
gaseous medium,
– the temperature, and the pressure.
• Spectral analysis shows line and
continuum emissions due to excited
and ionized states of atoms and ions
• Radiation  UV, visible, IR
10
10
• Energy loss due to radiation  10-20
%
• Highly hazardous to eyes, skin

6. 9/20/2012
6
Metal transfer in Arc welding
• The manner in which molten
filler metal is transferred to the
weld pool  profound effects
on the performance of a
consumable electrode arc
welding process
• These effects include
– Ease of welding in various
positions
– Extent of weld penetration;
11
p
– Rate of filler deposition and
– Heat input
– Stability of the weld pool
– Amount of spatter loss
Mode of Metal transfer-Influencing
parameters
• Pressure generated by the evolution of gas
at the electrode tip (for flux-coated or flux-cored
electrode processes)
• Electrostatic attraction between the
consumable electrode and the workpiece
• Gravity
12
y
• “Pinch effect” caused near the tip of the
consumable electrode by electromagnetic
field forces  spray

7. 9/20/2012
7
Mode of Metal transfer-Influencing
parameters
• Explosive evaporation of a necked region
formed between the molten drop and solid
portions of the electrode due to very high
conducting current density
• Electromagnetic action produced by divergence
of current in the arc plasma around a drop.
• Friction effects of the plasma jet (plasma
friction)
• Surface tension effects once the molten drop
(or electrode tip) contacts the molten weld pool
13
Metal transfer types
• Free-flight transfer: Complete
detachment of the molten metal
drop from the consumable
electrode  flight to the work piece
and weld pool, without any direct
physical contact
• Bridging transfer: molten metal
drops are never completely free;
rather they are always attached to
th bl l t d d th
14
the consumable electrode and the
workpiece, momentarily bridging
the two from a material standpoint -
and electrically
• Slag-protected transfer

8. 9/20/2012
8
Free-flight-Globular Transfer
• Low welding currents (50-170 A) in
pure argon, molten metal from a
small diameter solid steel wire
electrode is transferred in the form
of globules
• Drops diameter larger than the
wire
• Large drops  detach by gravity
15
• Low rate of globule formation,
detachment, and transfer (< 1-10
s-1)
• Globular transfer  down-hand
position
Free flight: Globular-projected
Transfer
• As the welding current increases
within the range of 50-170A, the
drops become progressively
smaller,  electromagnetic forces
are having an increasing effect on
detachment
• For DCEP, drop size 1/ welding
current
16
current.
• As welding current is increased,
the rate of drop transfer also
increases

9. 9/20/2012
9
Free-flight transfer modes
Increasing current
17
Individual drop formation and detachment sequence in (a) globular
transfer and (b) projected and (c) streaming axial spray transfer
Free flight -Spray
Transfer/streaming transfer
• At current > critical level  No
individual drops
• Tip of the consumable electrode
becomes pointed  cylindrical
stream of liquid metal flows toward
the work piece in line with the
electrode.
• Near its tip (nearest the work
piece) this cylinder disperses into
18
piece), many very small droplets 
Electromagnetic pinch effect
• The rate at which droplets are
transferred is hundreds per second.

10. 9/20/2012
10
Free flight -Spray Transfer:
Features
• Axial spray transfer mode  Excellent stability,
virtually free of spatter
• Droplets are actively propelled away from the
consumable electrode and into the molten weld
pool to be captured by surface tension force.
• Droplets are transferred in line with the electrode
 Filler metal can be directed exactly where it is
needed
19
needed.
• This is a great advantage when making vertical
or overhead welds, where the propelling force
offsets the disruptive effect of gravity
Bridging or short circuiting
transfer
• Large dia. Electrodes  too high
transition current to achieve axial spray
transfer (e.g., 200-220A for 1-mm-diameter
steel wire)  Bridging
transfer
• Voltage is kept low (say 17-21 V
versus 24-28 V for globular transfer
with steel wires)
20
• The tip of the electrode periodically
dipped into the molten weld pool.

11. 9/20/2012
11
Bridging or short-circuiting
transfer
• Bridging  Molten metal transfer by
a combination of surface tension and
electromagnetic forces
• Low voltage reduces the rate at
which the electrode is melted
• In the presence of carbon dioxide
(CO2) in shielding gas, the molten
drop at the electrode tip is pushed
21
p p p
upward  repelled transfer. In this
case, short-circuiting captures the
drop before it detaches in an
unfavorable manner
Short-circuiting transfer-
Advantages
• Less fluid molten metal (due to
less superheat)
• Less penetration (due to lower
welding voltage and lower net
energy input).
• Easy handling in all positions,
especially overhead and for the
22
overhead, joining of thin-gauge materials.
• Minimum Spatter

12. 9/20/2012
12
Sequence of short-circuiting
transfer
(a) globule of molten metal builds up on the end of the electrode; (b)
globule contacts surface of weld pool; (c) molten column pinches off to
detach globule; and (d) immediately after pinch-off, fine spatter may result
23
Short-circuiting transfer: I-V
trace
24

13. 9/20/2012
13
Pulsed arc or pulsed current
transfer
• Steady current to maintain the arc and a
periodic current pulse to a higher level
• Periodic pulses detaches a drop and
propels it into the weld pool,
• Advantage of axial spray transfer at a
lower average current, and, thus, lower net
heat input.
• The time period of pulses must be short
enough to suppress globular transfer, but
long enough to ensure that transfer by the
spray mode will occur
25
PULSED-CURRENT TRANSFER
26

14. 9/20/2012
14
Pulsed current transfer
• This pulsed mode differs from the normal spray
mode in that
– The molten metal transfer is interrupted between the
current pulses
– The current to produce spray is below the normal
transition current
• Pulse shape (i-e., wave form, especially the rate
of the rise and fall of current) and frequency can
be varied over a wide range in modern, solid-
27
g
state power sources
• Rate of molten metal transfer can be adjusted to
be one drop or a few drops per pulse (by
adjusting the pulse duration)
Classification of transfer modes
28

15. 9/20/2012
15
Dominant forces in transfer modes
29
Effect of welding process parameters
on transfer modes-Summary
• Current at which transition from globular to spray
transfer begins depends on
– Composition of the consumable electrode,
– Electrode diameter,
– Electrode extension,
– Composition of shielding gas.
• Shielding Gas Effects
30
• Process Effects
• Operating Mode or Polarity Effects

16. 9/20/2012
16
Welding Lecture ‐ 8
20 Sept 2012, Thursday 8.30 am ‐9.30 am
Welding Processes‐
Resistance welding
31
31
Resistance welding (RW)
• Generate heat through the
resistance to the flow of electric
current in parts being welded
• The parts are usually an integral
part of the electrical circuit
• Contact resistance  heats the
area locally by I2R,  melting 
formation of a nugget
C t t i t tb
32
32
• Contact resistance must be
higher at the point to be welded
than anywhere else.

17. 9/20/2012
17
Resistance welding
33
33
Resistance welding
34

18. 9/20/2012
18
Resistance welding
• Pairs of water-cooled copper electrodes
• Apply pressure
– To reduce the contact resistance at the electrode-to-workpiece
interface
– Contain the molten metal in the nugget
– To literally forge the work surfaces together in the vicinity of the weld
• The principal process variables
– welding current (several thousands to tens of thousands of amperes)
– welding time (of the order of s)
– electrode force and electrode shape
DC ( id d f ith i l h
• power provided from either single-phase or
three-phase AC line 440-480 V using step-down
transformer/rectifiers)
• Usually used to join overlapping sheets or plates as
lap joints, which may have different thicknesses
35
35
Resistance welding-types
• Resistance spot welding (RSW)
• Resistance seam welding (RSEW)
• Projection welding (PW)
• Flash welding (FW)
• Upset welding (UW)
36
36
• Percussion welding (PEW)

19. 9/20/2012
19
Resistance spot welding (RSW)
• Series of discrete nuggets
produced by resistance
heating
• Nuggets (welds) are
usually produced directly
under the electrodes, 
Not necessarily if there is
37
37
another more favourable
path (shunt), for the
current
Series resistance spot welding
38
38

20. 9/20/2012
20
Resistance welding cycle
• Squeeze Time: Time interval between timer initiation
and the first application of current needed to assure
that electrodes contact the work and establish full
force
• Weld time: The time for which welding current is
applied (in single impulse welding) to the work
• Hold Time: The time during which force is maintained
on the work after the last impulse of welding current
ends to allow the weld nugget to solidify and develop
strength.
• Off Time: The time during which the electrodes are off
39
the work and the work is moved to the next weld
location for repetitive welding.
Pressure-current cycle
40
40

21. 9/20/2012
21
Pressure-current cycle
1. Off time: Parts inserted between open electrodes,
2. Squeeze Time:Electrodes close and force is
applied,
3. Weld time— current is switched on,
4. Hold time: Current is turned off but force is
maintained or increased (a reduced current is
sometimes applied near the end of this step for
41
41
stress relief in the weld region), and
5. Off time: Electrodes are opened, and the welded
assembly is removed.
Enhanced welding cycle
1. Pre-compression force is used to set electrodes
and work pieces together
2. Preheat is applied to reduce thermal gradients at
the start of weld time or to soften coatings
3. Forging force is used to consolidate weld nugget
4. Quench and temper times are used to produce
desired weld properties in hardenable steels;
5. Post heat is used to refine weld nugget grain size
and improve strength
6. Current decay is used to retard cooling of
aluminum alloys to help prevent cracking
42

22. 9/20/2012
22
Enhanced
resistance
welding
cycle
43
Nugget formation
Nugget formation and
heat dissipation into the
surrounding base metal
and electrodes during
resistance spot welding
44

23. 9/20/2012
23
Optimum current, time in RW
45
• Optimum current and weld time for maximum shear
strength of the RW joint
Resistance seam welding
Conventional
C ti
46
46
resistance seam
welding, in which
overlapping spots
are produced
Roll spot
welding
Continuous
resistance
seam

24. 9/20/2012
24
Resistance seam welding
47
Mash seam weld
48
48

25. 9/20/2012
25
Projection welding (PW),
• Projections or dimples in overlapping joint elements 
concentrate the current during welding, focusing the weld
energy and helping to locate the weld more precisely
• Contact points determined by the design of the parts to
be joined  may consist of projections, embossments, or
localized intersections of the parts
49
49
Flash welding(FW)
1. Normally used for butt joints  the two surfaces
50
50
y j
to be joined are brought into near contact 
2. Electric current is applied to heat the surfaces to
the melting point  the surfaces are forced
together to form the weld

26. 9/20/2012
26
Upset welding (UW)
• Upset welding (UW) is
similar to flash welding
• Heating in UW is
accomplished entirely by
electrical resistance at
the contacting surfaces;
no arcing occurs.
• Not a fusion-welding
process
51
51
• Applications of UW &
FW- joining ends of
wire, pipes, tubes etc.
Percussion welding
• Resistance heating by the rapid release of electrical energy
from a storage device ( e.g., capacitor).
52
52
g g , p )
• Similar to flash welding, except that the duration of the weld
cycle is extremely short, ~ 1 to 10 ms
• Very localized heating  attractive for electronic
applications in which the dimensions are very small